It starts off with Adam and Eve outside the garden after they are condemned to multiply.

Which they immediately do and bear 2 sons. Cain and Abel

Now the fun part begins. Cain kills Abel and god punishes him to wander the earth.

13 Cain said to the LORD, “My punishment is more than I can bear. 14 Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me.”

15 But the LORD said to him, “Not so[e]; anyone who kills Cain will suffer vengeance seven times over.” Then the LORD put a mark on Cain so that no one who found him would kill him. 16 So Cain went out from the LORD’s presence and lived in the land of Nod,[f] east of Eden.

Now a few weird things get mentioned. Keep in mind at this point there were 4 people on earth now there are 3 as Abel is dead. Who is Cain worried will kill him while he is wandering? Where did the land of Nod come from? Why did god need to mark Cain? His parents know who he is.

So the next part of the story

17 Cain made love to his wife, and she became pregnant and gave birth to Enoch. Cain was then building a city, and he named it after his son Enoch. 18 To Enoch was born Irad, and Irad was the father of Mehujael, and Mehujael was the father of Methushael, and Methushael was the father of Lamech.

Where did Cains wife come from? Its not his sister because Adam and Eve dont procreate again until verse 25

25 Adam made love to his wife again, and she gave birth to a son and named him Seth,[h] saying, “God has granted me another child in place of Abel, since Cain killed him.” 26 Seth also had a son, and he named him Enosh

Where might we find the cliff-notes to know which ones are metaphorical? And in what year did the become metaphorical. My Spidy-sense says that you'll find the metaphors begin when science has back the teachings into a corner. But prior to that, exact words of God.

Well there is The twin genetic stumps of Adam & Eve and Noah and family.

Which of course, the incest, is implied by the absurdity of 2 or 8 people populating the planet with all it's diversity- same as 2 animals associated with Noah- the numbers are absolutely preposterous.

As for actual incest:

Lot had sex with both of his daughters AND got them pregnant Genesis 19:30-38...

The daughters got him SO drunk he didn't remember---but he could still get an erection.

Of course this was on two consecutive nights as well, one on each night- should've had a 3-some. I mean as long as he's going as far as incest in a drunken haze...

Whoa Nelly! I am thinking you are wrong there old pard. Except for some intellectual Christians in some of the larger cities, and maybe Jews, the Bible was taken literally up until 50 to 100 years ago by most of the faithful. I heard more than once, growing up that the entire Bible was "The Word of God" and wasn't to be questioned. If you had any doubts, you needed a faith injection. There were lessons to be learned, sure, but there was no metaphorical about any of it!

Read Rick Warrens book "The Purpose Driven Life". In it he writes that God (in His infinate wisdom) gives each of the off-spring of Adam and Eve, who were Caucasian, gave birth to Cain, who was Oriental and Seth, who was black, etc. vastly different DNA. This man has single-handedly taken Christian apologism to a whole new level.

just because it doesn't mention them having more kids until seth doesn't mean they didn't. the authors objective is not those details. cain's wife probably was his sister. the point of the geneologies is comparing cain's to seth's which leads to noah, other kids they had in the mean time isn't the author's concern. the land of nod was probably a later insertion so that the audience for whom it was intended would know the area based on a name they would recognize.

- If you just mean the other local goat hearders- if we are to belive the "geneologies"- even in the Land of Nod- they are all close reletives. 2nd, 3rd, and 4th cousins, all of course also inbred and getting progressively more so with each generation.

With the passage of enough time you will get some relatives far enough removed that they might be able to have children that have a full set of teeth again and not spit tobacco while Dueling Banjos plays in the background...